I've used various methods over the years to punch holes in my exhausted iso/propane canisters, but this neato little tool from JetBoil makes the job quick, easy, and efficient. Being able to thread the collar onto the canister itself lets one do the job without having to stab or put out an undue amount of pressure.

This is handy when it's raining or snowy out, and is a bit safer than using a hunting knife. Ask me how I know.

I'm always a popular guy at the end of group camping trips when I pull this out. :)